Applied in process- and/or automation technology are many different field devices for determining and/or monitoring at least one process variable, especially a physical or chemical, process variable. Involved in such case, are, for example, fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, and conductivity, respectively. A field device typically includes at least one sensor unit, which at least partially and at least at times comes in contact with the process, and an electronics unit, which serves, for example, for signal registration,—evaluation and/or—feeding. Referred to as field devices in the context of the present invention are, in principle, all measuring devices, which are applied near to the process and which deliver, or process, process relevant information, thus also remote I/Os, radio adapters, and, generally, electronic components, which are arranged at the field level. A large number of such field devices are produced and sold by the firm, Endress+Hauser. Applied for registering the respective process variables are many different measuring principles, which are known from a large number of publications. To discuss separately and in detail each field device, for which the solution of the invention is appropriate, would be more than necessary. Thus, the following description focuses on fill level measuring devices with an oscillatable unit, wherein, at suitable locations, reference is made to other possible applications of the solution of the invention.
Such field devices, also referred to as vibronic sensors, have, especially in the case of fill-level measuring devices, for example, an oscillatory fork, single rod or membrane as the mechanically oscillatable unit. This is excited during operation by means of a driving/receiving unit, usually in the form an electromechanical transducer unit, to execute mechanical oscillations. The transducer unit can be, for example, a piezoelectric drive or an electromagnetic drive. In the case of flow measuring devices with at least one oscillatable unit, such can, however, also be embodied as an oscillatable tube, through which the medium flows, an example of such being a measuring device working according to the Coriolis principle.
Corresponding field devices are produced by the applicant in great variety and, in the case of fill-level measuring devices, sold, for example, under the marks, LIQUIPHANT and SOLIPHANT. The underpinning measuring principles are basically known. On the one hand, the driving/receiving unit excites the mechanically oscillatable unit by means of an electrical excitation signal to execute mechanical oscillations. On the other hand, the driving/receiving unit receives the mechanical oscillations of the mechanically oscillatable unit and converts them into an electrical, received signal. The driving/receiving unit is, correspondingly, either a separate drive unit and a separate receiving unit, or a combined driving/receiving unit.
For exciting the mechanically oscillatable unit, the most varied of both analog as well as also digital methods have been developed. In many cases, the driving/receiving unit is part of a feedback, electrical, oscillatory circuit, by means of which the exciting of the mechanically oscillatable unit to execute mechanical oscillations occurs. For a resonant oscillation, for example, the amplification factor must be ≥1, and the oscillatory circuit condition, according to which all phases arising in the oscillatory circuit add up to a multiple of 360°, must be fulfilled.
For fulfilling the oscillatory circuit condition, a certain phase shift between the excitation signal and the received signal must be assured. The most varied of solutions are known for this. In principle, the adjusting of the phase shift can be accomplished, for example, using a suitable filter, or also by means of a control loop controlled to provide a predeterminable phase shift, the desired value. Known from DE102006034105A1, for example, is use of a tunable phase shifter. The additional integration of an amplifier with adjustable amplification factor for additional control of the oscillation amplitude is, in contrast, described in DE102007013557A1. DE102005015547A1 uses an all-pass filter. The adjusting of the phase shift is, moreover, possible by means of frequency search operation, such as, for example, disclosed in DE102009026685A1, DE102009028022A1, and DE102010030982A1. The phase shift can, however, also be controlled to a predeterminable value by means of a phase locked loop. Such an excitation method is subject matter of DE00102010030982A1.
Both the excitation signal as well as also the received signal are characterized by frequency, amplitude and/or phase. Changes in these variables are usually taken into consideration for determining the particular process variable, such as, for example, a predetermined fill level of a medium in a container, or also the density and/or viscosity of a medium or in the case of a flow measuring device the flow of a medium through a tube or pipe. In the case of a vibronic limit level switch for liquids, it is, for example, distinguished, whether the oscillatable unit is covered by the liquid or freely oscillating. These two states, the free state and the covered state, are, in such case, distinguished, for example, based on different resonance frequencies, thus a frequency shift.
Density and/or viscosity can, in turn, be ascertained with such a measuring device only when the oscillatable unit is covered by the medium. Known from DE10050299A1, DE102006033819A1 and DE102007043811A1 is to determine the viscosity of a medium based on the frequency-phase curve (Φ=g(f)). This procedure is based on the dependence of the damping of the oscillatable unit on the viscosity of the involved medium. In order to eliminate the influence of density on the measuring, viscosity is determined based on a frequency change caused by two different values for the phase, thus by means of a relative measurement.
For determining and/or monitoring the density of a medium, in contrast, according to DE10057974A1, the influence of at least one disturbing variable, for example, the viscosity, on the oscillation frequency of the mechanically oscillatable unit is ascertained and compensated. In DE102006033819A1, it is, furthermore, disclosed to set a predeterminable phase shift between the excitation signal and the received signal, in the case of which effects from changes of the viscosity of the medium on the mechanical oscillations of the mechanically oscillatable unit are negligible. At this phase shift, an empirical formula for determining the density can be set up.
In order to register different process variables with a certain field device, thus, in each case, different embodiments and methods must be applied for operating the field device. Even when a number of process variables are determinable by means of a field device, in many cases the determining of a first process variable must be interrupted, in order to be able to determine a second process variable. It would, however, be desirable, when by means of one field device a number of process variables would be simultaneously and continuously determinable without mutual influencing.